Ecology Of The Pelagic Larvae Of Coral Reef Fishes

In common with many marine animals, fishes that live on coral reefs have a two-part life history: a relatively sedentary adult phase on the reef, and a potentially very mobile pelagic larval phase in open water. One major question our research seeks to answer is "where do the larvae spend their pelagic period".

In common with many marine animals, fishes that live on coral reefs have a two-part life history: a relatively sedentary adult phase on the reef, and a potentially very mobile pelagic larval phase in open water. Adult reef fishes take little or no care of their young: most larvae end up off the reef into open water where they are left to fend for themselves for anywhere from two to 20 weeks before they must find a coral reef to settle on. Little is known of the biology of these tiny (typically, 1-20 mm long) fishes during this pelagic period in open water. Because most dispersal takes place during the pelagic larval phase, it is important to both researchers and managers to know what the larvae are doing, where they are doing it, and how far they move during the pelagic phase.

Distributional Ecology Of Reef-Fish Larvae

One major question our research seeks to answer is "where do the larvae spend their pelagic period". Work at the Australian Museum's research station at Lizard Island on the Great Barrier Reef and in the south Pacific islands of French Polynesia is aimed at answering this question. We now know, for example that larvae of some fishes normally remain within a few hundred metres of where they were spawned, that others can complete their pelagic stage in the lagoons of coral atolls, but that the vast majority apparently require more-or-less open waters during their pelagic phase.

This research has been supported by the ARC, the Commonwealth Department of Industry, Science and Technology, the Australian Museum and an international collaborative research program with French scientists ('PICS'). External collaborators include: Professor J Howard Choat of James Cook University, Townsville; Dr Peter J Doherty of the Australian Institute of Marine Science, Townsville; and Drs Rene Galzin and Vincent Dufour of the Universite de Perpignan, France. Australian Museum staff collaborators include Ms Sally E Reader and Dr Tom Trnski.

Abstracts of recent papers from this project:

What does Larval Fish Biology Tell Us About the Design and Efficacy of Marine Protected Areas?

Abstract
Marine Protected Areas (MPAs) can theoretically achieve two Goals: protection of biodiversity, and replenishment of populations both inside and far outside the MPA boundary. The second is supposed to result primarily from larval export from the MPA. Although there is evidence that 'no take' MPAs protect biodiversity and have higher stocks of larger, older, more fecund fishes, there is scant empirical evidence to support the notion that MPAs actually do replenish unprotected areas, or if they do, over what spatial scale. This notion of replenishment over large scales is largely based on theoretical considerations of larval dispersal and larval biology. Recent research shows that at least fish larvae do not conform to traditional theory: they may have much more control over where they disperse than previously thought. This has important implications for the design and implementation of MPAs, and what we can expect from them as conservation tools. This paper reviews recent advances in understanding larval fish biology and behavioural capabilities and how these impact on the efficacy and design of MPAs. If larvae are as good at resisting dispersal as their behavioural capabilities suggest, then replenishment in ecologically meaningful quantities probably takes place over much smaller scales than previously thought, and MPAs will have to be designed accordingly. These scales, however, are likely to differ spatially, temporally and among species.
Published in 2003. Pp 170-180 in: J.P. Beumer, A. Grant and D.C. Smith, (eds) Aquatic Protected Areas: What works best and how do we know? Proceedings of the World Conference on Aquatic Protected Areas, Cairns, August 2002. Australian Society for Fish Biology, North Beach, WA

Local completion of the pelagic larval stage of coastal fishes in coral-reef lagoons of the Society and Tuamotu Islands.

Abstract
In 4 lagoons at 2 atolls and 1 high island in the Tuamotu and Society Islands, French Polynesia, plankton samples were taken weekly on 4 weeks in January/February 1989. A third atoll lagoon was sampled once. The lagoons varied in size and physical openness. We also sampled in the ocean near 2 atolls and the high island. All locations were sampled during the day, and 3 lagoons (2 atolls and 1 high island) were also sampled at night. Pelagic fish eggs were more abundant in the ocean than in the lagoons at the atolls, but not at the high island. Larvae of coastal fishes were abundant in all lagoons. In the atoll lagoons, larvae of oceanic fishes were very rare to absent, but in the high-island lagoons and in the ocean, they were commonly encountered. In the ocean, larvae of many typical reef-fish taxa were abundant (58 taxa were represented by at least 10 individuals), but in the lagoons, most of these were rare or absent, and we conclude these rare and absent taxa normally do not complete their larval phase in lagoons.

Taxa were considered to be able to complete their pelagic phase in a lagoon (ie, were 'completers') if they were present in the lagoon plankton samples from across a full larval size range. In the high-island barrier-reef lagoon, young, preflexion larvae were abundant, but only 2 taxa (of 56 captured) were present over a wide size range and were considered completers in this lagoon. In the high-island lagoonal bay, 11 taxa (of 67 captured) were considered completers. The numbers of taxa captured in the 3 atoll lagoons ranged from 39-44, and the number of taxa considered completers increased with increasing lagoon size and physical openness. The 17 completer taxa in the smallest, most enclosed atoll lagoon were, with 1 exception, a subset of those (18) in the second lagoon which, in turn, with 1 exception, were a subset of those in the largest, most open lagoon (26). Completer taxa were of the families Apogonidae, Blenniidae, Bothidae, Callionymidae, Carangidae, Gobiidae, Microdesmidae, Mullidae, Pomacentridae, Schindleriidae and Tetraodontidae. The species that can complete their pelagic periods in coral-reef lagoons are a highly predictable group, and not simply a random selection of the potential species pool. Most of these species hatch from non-pelagic eggs. Water renewal times in the atoll lagoons, unlike the high-island barrier-reef lagoon, were much longer than expected pelagic larval durations of completer taxa. Demographically, lagoon populations of completer taxa apparently self-recruit and are probably near the closed end of the open/closed population continuum. The lagoonal bay on the high island differs from the other lagoons in containing larvae of species not found elsewhere, including some completers, and lacking some species that are abundant completers in other lagoons. In French Polynesia, lagoon size is a strong predictor of the number of lagoon completer taxa. The number of completer taxa apparently peaks at intermediate lagoon water-exchange times.

The following paper reviews developments in this research field:
Leis, J. M. and M.I. McCormick. 2002. The biology, behaviour and ecology of the pelagic, larval stage of coral-reef fishes. P171-199 In P.F. Sale, (ed). Coral Reef Fishes: Dynamics and diversity in a complex ecosystem. Academic Press, San Diego.